What's the problem with the port 'Hilo'? Why couldn't they just be sailing to/from Hilo? Plenty of other ports named in shanties.

If you're looking for origins of hi/hy/high/ low/lo you're going to have an awful lot to go at. I can't find any in nautical slang dictionaries but 'high-lows' were laced boots c1801 onwards. High and Low separately have an awful lot of synonyms.

I'm sure your right that it came from the field hollers in some cases but it could quite easily have 2 usages.